fix(kv-cache): unblock per-side ROT_OVERRIDE knobs (default attn_rot_disable=false)

Previous state: db3595a added LLAMA_ATTN_ROT_K_OVERRIDE / _V_OVERRIDE per-side
opt-in knobs but kept attn_rot_disable defaulting to TRUE for legacy
LLAMA_ATTN_ROT_DISABLE compatibility. The override branches included
`&& !attn_rot_disable` guards, so when LLAMA_ATTN_ROT_DISABLE is unset
(default true) the per-side env knobs were silently no-ops. Users could not
opt into rotation without also setting LLAMA_ATTN_ROT_DISABLE=0.

Fix: flip attn_rot_disable default to false. Rotation is still OFF by default
because attn_rot_k/v default to false. LLAMA_ATTN_ROT_DISABLE=1 still acts as
a hard lock-out that blocks the per-side overrides for users who want a
single switch to guarantee no rotation.

Caught while running the cross-format KLD matrix for the rotation/PPL
investigation paper — V-only override appeared to silently fail. Confirmed
with logs that attn_rot_v stayed 0 even with LLAMA_ATTN_ROT_V_OVERRIDE=1
until this default flip.

Author: TheTom

src/llama-kv-cache.cpp

@@ -489,8 +489,12 @@ llama_kv_cache::llama_kv_cache(
     //
     // LLAMA_ATTN_ROT_DISABLE retained as a no-op alias (default OFF makes it
     // redundant but historical scripts may set it).
+    // Default attn_rot_disable=false now that rotation is OFF by default. The
+    // env var is preserved as a hard lock-out (=1 forces rotation off and
+    // blocks overrides), useful for users who want to guarantee no rotation
+    // regardless of any LLAMA_ATTN_ROT_*_OVERRIDE settings.
     const char * LLAMA_ATTN_ROT_DISABLE = getenv("LLAMA_ATTN_ROT_DISABLE");
-    const bool attn_rot_disable = LLAMA_ATTN_ROT_DISABLE ? atoi(LLAMA_ATTN_ROT_DISABLE) : true;
+    const bool attn_rot_disable = LLAMA_ATTN_ROT_DISABLE ? (atoi(LLAMA_ATTN_ROT_DISABLE) != 0) : false;
 
     // Default: rotation OFF on both sides (safe across all tested model families).
     // Override per side via env vars below.